Open loop multi-reflection time of flight mass spectrometer are usually composed of repeat focusing cells in which ideal stigmatic focusing in X-, Y- and Z-dimensions is achieved from cell to cell. Cells can either be segments of sectors, quadrupole devices, Einzel lenses or combinations of these devices. Typically, it is a requirement that the angular and lateral magnification for each dimension is as close to unity as possible through each cell, or through integer multiple of cells. If the focusing magnification is not unity, then for each circuit of the ion beam, the beam dimensions would iteratively expand beyond the geometric limits of each focusing device and ions would be lost.
In addition to geometric focusing, it is also a requirement to have a good degree of energy focusing. This is usually achieved by higher energy ions taking an extended flight path through each reflecting device. Although these higher energy ions have a relatively long time of flight through each reflecting device, this is balanced by the relatively shorter time of flight of these ions through the field-free regions.
It is well known that the mass resolving power of a time of flight mass spectrometer can be increased by extending the overall flight path for all of the ions, provided that the stigmatic and energy focusing aberrations are minimised over the complete flight. However, as the ion flight path length is increased the ions become proportionally more susceptible to collisions with residual gas molecules. Such collisions cause scattering of ions and huge losses in ion transmission and instrument resolution. As such, a relatively high vacuum must be maintained in the instrument. It is particularly difficult to include a relatively high pressure gas cell within or downstream of the time of flight instrument without causing an undesirably high collision rate within the ions time of flight path. For example, it is particularly difficult to include a relatively high pressure collisionally induced dissociation (CID) cell within such an instrument for performing MS/MS analysis.
It is desired to provide an improved mass spectrometer and an improved method of mass spectrometry.